1. Magazine, not clip.
2. Redundant systems is a good idea.
3. Buy the same model of pistol, use the same magazines.
4. HK's are damn tough. I've got one and have shot the piss out of it over the past 8 years. Still running strong.
5. No other pistols use the HK magazines that I know of. HK mags are retained by a tab on the front of the magazine rather than a notch on the side as most used to have. More and more are going to ambi releases and using the front tab method, but HK mags are HUGE. Generally speaking, they make the mags for the gun. The only reason that some guns like Beretta 92's and Taurus 92 are interchangeable is because it's a copy of the same system.